P M V Subbarao Professor Mechanical Engineering Department Superheating of Steam P M V Subbarao Professor Mechanical Engineering Department Recognition and Adaptation of Efficient Mode of Heat Transfer …..

Furnace Energy Balance Enthalpy to be lost by hot gases: Water walls Economizer Furnace

Capacity of Flue Gas Total Thermal Power available with flue gas: Rate of steam production:

3 5 2s 2f 4 2 1 6 s

Paths of Steam and Gas Drum Water walls Economizer

Thermal Structure of A Boiler Furnace DPNL SH Platen SHTR R H T LTSH Economiser APH ESP ID Fan drum Furnace BCW pump Bottom ash stack screen tubes

Convective Superheater (Pendant) Convective super heaters are vertical type (Pendant ) or horizontal types. The Pendant SH is always arranged in the horizontal crossover duct. Pendant SH tubes are widely spaced due to high temperature and ash is soft. Transverse pitch : S1/d > 4.5 Longitudinal pitch : S2/d > 3.5. The outside tube diameter : 32 – 51mm Tube thickness : 3 – 7mm S1 S2

Convective Superheater (Horizontal) The horizontal SH are located in the back pass. The tubes are arranged in the in-line configuration. The outer diameter of the tube is 32 – 51 mm. The tube thickness of the tube is 3 – 7 mm. The transverse pitch : S1/d = 2 – 3. The longitudinal pitch :S2/d = 1.6 – 2.5. The tubes are arranged in multiple parallel sets. The desired velocity depends on the type of SH and operating steam pressures. The outside tube diameter : 32 – 51mm Tube thickness : 3 – 7mm S1 S2

Thermal Balance in Convective SH. The energy absorbed by steam The convective heat lost by flue gas Overall Coefficient of Heat Transfer, K

Mean Temperature Difference The average temperature difference for parallel flow and counter flow is expressed as It is also called log mean temperature difference When Dtmax / Dtmin > 1.7, the average temperature may be expressed as: Generally, the flow direction of the flue gas is perpendicular to the axes of tubes. If number of bends are more than four, the flow can be treated as counter or parallel flow.

S1 S2 Inline S1 Staggered S2

Effectiveness Factor The ratio of the heat transfer between fouled and clean tubes.

Thermal Balance in Super Heater. The energy absorbed by steam The convective heat lost by flue gas Overall Coefficient of Heat Transfer, U Platen SH, U (W/m2 K) 120 – 140 Pendent SH, U (W/m2 K) Convective SH, U (W/m2 K) 60 – 80

Reheater The pressure drop inside reheater tubes has an important adverse effect on the efficiency of turbine. Pressure drop through the reheater should be kept as low as possible. The tube diameter : 42 – 60mm. The design is similar to convective superheaters. Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.